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Bulletin 420

February, 1939

The Native Elm Bark Beetle

Hylurgopinus rufipes (Eichhoff)

In Connecticut

B. J. Kaston

J^gnculiural ^Experiment Jitaium

H^ui iHauen

CONTENTS

Introduction 3

Life History and Harits

Emergence of Adults from Hibernation 3

Oviposition 3

Larval Period 8

Pupal Period 18

Emergence of Adults from Bark 20

Flight and Wind Carriage • 22

Bark Tunnels in Living Elms 23

Hibernation 28

Number of Generations 29

Natural Factors of Control 31

Associated Fauna 36

Summary 36

Bibliography 38

The Native Elm Bark Beetle

Hylurgopinus rufipes (Eichhoff)
in Connecticut

B. J. Kaston

Because of their importance in the dissemination of the fungus causing
the Dutch elm disease, the habits of elm bark beetles have been studied
by a number of investigators during the past few years. In Connecticut,
investigations have been carried out particularly on the native elm bark
beetle, Hylurgopinus rufipes Eichhoff. This beetle occurs throughout the
entire State and is much more abundant than the European elm bark
beetle, Scolytus multistriatus Marsham. Besides the then known distribu-
tion, as given in a previous paper (Kaston, 17), the species has since been
recorded from Rhode Island, New Hampshire (Collins, et al, 8), Alabama,
and Mississippi (Collins, 7). Its distribution is particularly significant
when considered with the fact that a number of Dutch elm diseased trees
have been found in outlying areas where the European beetle does not occur.
These include Old Lyme, Conn.; Cleveland, Ohio; Baltimore, Md.; and
Norfolk, Va.

Certain taxonomic considerations and a discussion of the morphology
have already been published (Kaston, 17). The writer is indebted to
Mr. W. 0. Filley and Dr. R. B. Friend under whose supervision the
investigations were conducted. He also wishes to acknowledge very con-
siderable assistance from Mr. D. S. Riggs, whose aid both in the field and
in the laboratory was practically indispensable. The photographs are the
work of Mr. B. II. Walden and the drawings of Mrs. Elizabeth Kaston.

LIFE HISTORY AND HABITS

, Emergence of Adults from Hibernation

If bark tunnels in which beetles hibernate are investigated during late
April and early May, when the trees begin to leaf out, it will be found that
they contain fresh boring dust. The beetles are active when disturbed
and may even crawl out of the tunnels to walk about over the surface of
the bark. Usually, however, the beetles dig further in the old tunnel before
leaving to attack breeding material. By the latter part of May all hiber-
nating tunnels are deserted, and beetles may be seen walking about over
trap logs and attacking other attractive material.

Oviposition

Suitable material for oviposition may include any dying or dead (but
not too dry) elm limb about two inches in diameter or larger. Occasionally
a smaller limb, and quite often the trunk of a tree, may be attacked. In
the latter case, if only one or two leaders are dying, the part of the trunk

Connecticut Experiment Station

Bulletin 420

£". tfASTWf

Figure 1. Semi-schematic drawing of a portion of a log. The bark is repre-
sented as having been cut through two entrance galleries, each with its accumula-
tion of boring dust in bark crevices. Another entrance hole is evidenced by this
frass accumulation on the bark near the left border. A short gallery with eggs
and young larvae is shown near the lower right corner of the exposed wood surface.
At the top of the exposed wood surface is a set of galleries, and to the left of this
an almost fully developed family. Several occupied pupal cells are shown on
the cut bark surface and an empty cell with an exit hole is inclined on the lower
left. Several other exit holes can be seen. (Footnote continued at bottom of
page 5.)

Life History and Habits 5

on which these grew may be attacked. Then there is usually a sharp de-
marcation between the portion still healthy, and the dying zone, with
beetles restricted to the latter.

Trap logs were cut from healthy trees and set out in various localities.
These were usually from 28 to 36 inches long, and from 3 to 8 inches in
diameter. The localities selected were those in which elms grew in numbers,
usually along streams and in swamps. Some were placed in deep shade and
others where they could get full sunlight. Most of the logs were propped
against the trunks of trees, but some were placed horizontally either on
the ground or on a rack support.

Although logs were cut at intervals throughout the summer, very few
beetles entered in August and September. Most of the attacks occurred
in May and June. This agrees with the findings of Martin (24), who com-
mented on the apparent paradox of the large numbers of young adults
emerging from spring-laid eggs and their not entering suitable breeding
places. This can be explained by the fact that the majority of newly
emerging beetles first make bark tunnels in healthy trees. A discussion of
this matter is reserved for a later section of this bulletin.

Several logs cut in the summer of 1935 and not attacked that year
were left lying over until the succeeding year when they were attacked at
the same time as the newly cut logs. Examination showed the bark to be
quite moist and fresh looking.

The attractiveness of a log seems to be intimately related to its water
content. Those left lying without any shade, where the heat of the sun
could dry them rapidly, did not attract beetles. The undersides of the logs
were more often attacked than the upper. In fact, the upper sides were
seldom attacked unless in deep shade.

In one experiment a number of logs were weighed immediately after
cutting and stored under diverse conditions for different lengths of time.
After having lost weight in various amounts, they were weighed again
when placed out to be attacked by beetles. Other factors being about
equal, it was found that beetles did not enter logs which had lost more
than 20 percent of their original weight. This is approximately half of the
original water content, as determined by oven-drying samples from a
freshly cut, healthy tree. These samples were cut from the lower trunk,
middle trunk region, and the top of the tree, and included a complete disc
of bark and wood. The average loss was 41 percent of the original weight.
In this connection it may be mentioned that Martin (24) reported there is
no correlation between the moisture content of the phloem and Hy-
lurgopinus (and Scolytus) infestations. He did not explain just how the
water content of this region was determined.

In addition to trap logs, several trees about 6 to 8 inches in diameter
at breast height were girdled in 1935, in an attempt to make them recep-
tive to beetles. Martin reported that his girdled trees were not infested,
but he failed to state whether or not the trees had died. In our experience
with 11 trees only one died the same season as girdled, and it was attacked
by large numbers of beetles. Ten leafed out in 1936 and five of these died

In the center of the lower part of the bark a portion of the outer layers is removed
to expose bark tunnels. (Note: Of course all these would not be seen at the
same time in one log. Moreover, larval tunnels are not as conspicuous on the
wood surface as indicated here.)

6 Connecticut Experiment Station Bulletin 420

that year. The remaining five leafed out and died in 1937. After they died
and before they dried out, these trees became attractive to beetles and
large broods successfully developed in them.

Entrance gallery

The entrance tunnel of the adult beetle naturally varies in length with
the thickness of the bark. The beetle selects a crevice in the bark or may
get under an overhanging flap. Generally these tunnels are cut perpen-
dicular to the outer surface and go directly into the wood surface (Figure 1).
Often, however, they are inclined and approach the wood surface at an
angle.

There seems to be no rule about which sex starts the gallery. Of 101
entrances examined during a period of about five weeks, 40 were being
made by males and 61 by females. In addition, on five occasions "paired"
beetles in entrance tunnels turned out to be two males, and on four occa-
sions two females. There were also six instances of three beetles in one
entrance hole. It would seem as if the eventual matching up of the sexes
was a matter of mere chance. Some start entrance holes later to be joined
by an individual of the opposite sex that has been wandering about over
the log. After the egg gallery is started the normal condition is estab-
lished, i.e., the male nearer the entrance of the tunnel.

Egg gallery

Upon reaching the wood surface, the parent beetle constructs the egg
gallery. Most commonly a biramous gallery is cut with the arms extending
away from the entrance tunnel at various angles. While this egg gallery
may be quite horizontal, it should be emphasized that more often it is
inclined from the horizontal. Usually in a biramous gallery the inner end of
the entrance tunnel forms the bottom of a "V" made by the two arms (Fig-
ure 1), though these may come off at any angle, independently of one
another, and are seldom equal in length. In our studies on large numbers
of galleries we found the biramous type most common, as indicated in
Table 1. In some cases there is a shallow pit in one of the arms where the
beetle has deepened the floor of the gallery. This is probably used as a
turning place, as is also the entrance tunnel. The junction of the latter
with the egg gallery probably also serves as a nuptial chamber. The egg
gallery of this species scores the wood very slightly, and at times not at
all, the parent then building entirely in the bark.

Table 1. Types of Egg Galleries Found in Random Sample of 1745, all at
Least 10 Millimeters in Length

Type Number Percent

Uniramous 221 12.5

Biramous 1428 82. 0

Triramous 89 5.1

Quadriramous 7 0.4

Life History and Habits 7

The author has never been successful in seeing the mating of this
species, despite thousands of galleries exposed in the course of this study.
By analogy with other bark beetles one would expect that copulation takes
place at the junction of the entrance tunnel and egg gallery. To determine
when copulation had taken place, females were selected at different stages
in the building of their egg galleries and dissected to see whether or not
there were spermatozoa in the seminal receptacles. In this way it has been
shown that mating may take place even before the building of the egg
gallery. In general, however, the number of females with spermatozoa
increases with the length of the gallery. In galleries of 10 millimeters or
longer, all the females contained spermatozoa.

Figure 2. An example of a young

egg gallery in which the eggs have been

laid quite close together, (x 4).

The length of the egg galleries and the number of eggs per gallery varies.
We have noticed that completed galleries in thick bark, as e.g., on trunks,
are longer than those in thinner bark. The longest seen was 77 millimeters,
but the average appears to be 30 millimeters. Eggs may be laid closely
packed on both sides of the gallery (Figure 2). Since the short diameter of
the oval egg is about 0.4 millimeters, a maximum of five eggs per milli-
meter of gallery length (along both sides) is physically possible. This
would allow very little space for packing frass, and ordinarily eggs are not
packed so closely. The greatest density observed was 44 eggs in 9 milli-
meters of gallery, or 4.89 eggs per millimeter. This gallery was about
11.5 millimeters long at the time, but, as is usual, eggs were not laid within

8 Connecticut Experiment Station Bulletin 420

2.5 millimeters of the entrance tunnel. Since frequently there are consid-
erable spaces in which the female lays no eggs, the average density appears
to be about two eggs per millimeter. Thus the average number per gallery,
or per female, is about 60. This approximates the average found by
actual count.

When beetles are abundant in the vicinity of suitable logs, many enter
in a very small area. In especially attractive places on a log we have
counted as many as 21.8 egg galleries per square decimeter over an area
of 4.04 square decimeters. While this is exceptionally high, we have also
counted 769 galleries on a log whose surface area was 47.32 square deci-
meters, or an average of about 16.3 per square decimeter. The average of
14 logs of assorted sizes which were used for complete counts, including
emergence of the brood, was 4.4 per square decimeter. This would seem to
be a moderately high average for attractive logs. From random sampling
of trees in the field, the average would appear to be nearer 2.5 per square
decimeter.

Rate of oviposition and period of incubation

Allowing three or four days for making the entrance tunnel, and an-
other three or four for the first part of the egg gallery, the first egg is laid
about a week after the entrance tunnel is begun. The female continues to
lay as she extends the tunnel in length. As many as six eggs may be laid
in one day, but the average is probably three or four during late June and
early July. Because of the practical difficulty involved, we were unable to
make actual observations on this process under normal conditions; hence
we resorted to the following method. Many pieces of bark containing
galleries with actively laying females were removed from the wood and
placed in the constant temperature room. We noted the number from each
family that hatched on any one day and assumed that these had probably
been laid the same day. This, of course, is not strictly accurate, as it does
not take into account the temperature at the time of laying. Moreover, it
tends to give a low figure, for some eggs did not hatch. Hatching occurs
in five or six days at a temperature of 25° C.

To determine whether or not females lay two sets of eggs the same
season, logs containing adults with galleries having young larvae were
caged with freshly cut logs. At intervals the logs were sampled to note the
progress of the brood. At the time when the first pupae were beginning to
transform to the adult stage, the fresher logs were removed and the bark
peeled and examined for galleries. None were found in them. Moreover,
most of the galleries in the old logs still had their parent beetles, and some
of the latter were dead and moldy. These observations indicate that a
female lays a single set of eggs a season, and that the parent beetles die
that season. The discovery of egg galleries from which the parent beetles
are missing cannot be construed as evidence that the beetles have gone
elsewhere to lay. Rather, the possibility is strong that the parents have
been killed and eaten by predators, such as, for example, Enoclerus ni-
gripes Say.

Larval Period

When the larva hatches, it proceeds to feed in a line approximately at
right angles to that of the egg gallery. Ordinarily this means parallel with
the grain, but later it may deviate from that direction (Figure 1)..

Life History and Habits 9

Digestion of cellulose

An attempt was made to ascertain whether or not the larvae could
digest the cellulose present in the bark (and wood). Two techniques were
employed. In one the gut was dissected out from each actively feeding
larva, placed in a depression slide and teased apart in a drop of water.
A minute drop of toluene for preservative and a single fibre of ash-free
filter paper as substrate were placed in the mixture. The whole was then
covered and sealed to prevent drying out. The slides were incubated at
about 35° C. and examined each day. Of 20 such preparations observed
over four days none showed any digestion of the cellulose fiber.

.268 .324 .409 .493 .578 .663 .747 .832 .917 .959

WIDTH OF HEAD CAPSULE in mm.

Figure 3. Frequency distribution of the head measurements of 853 larvae.

These are mixed lots taken at random from various localities in the field. Each

division on the abscissa equals .014 mm. (From Kaston and Riggs.)

In the second method the larvae were all ground up together in 10
parts by volume of 80 percent glycerine. The mixture was filtered at room
temperature and only that filtrate used which came through in the first
24 hours. The mixture was then diluted to about 5 percent glycerine solu-
tion by adding an acetate buffer of pH 5. The resulting mixture was poured
over pieces of ash-free filter paper and incubated at 35° in closed petri
dishes. There was no digestion of the cellulose by this method either.
It would seem, therefore, that the beetles had to derive their nutriment
from the other constituents of the bark, the cellulose apparently passing
through the digestive system unchanged.

Larval activities

In attempting to ascertain the number of larval instars, we first resorted
to measuring the head widths of mixed lots of larvae taken at random from
various localities in the field, as reported by Kaston and Riggs (21).
This method has been successfully used by Blackmail (4) on Pityogenes

Connecticut Experiment Station

Bulletin 420

hopkinsi Swaine, by Prebble (28) on Ips pini Say, Pityokteines sparsus
Leconte and Dendroctonus simplex Lee, and by Bedard (3) on D. pseu-
dolsugae Hopkins.

Larvae preserved in alcohol were measured to the nearest division of an
ocular micrometer. They ranged from 19 to 68 units, or, as one division
equals .014 millimeter, from .268 to .959 millimeters. The series of measure-
ments of 863 larvae collected during the first season (1934) is shown in
Figure 3. There is no distinct indication of a series of separate instars.
The first peak is definitely known to represent the first instar, for many of
these measurements were made upon larvae which had just hatched from
the egg. From measurements made on specimens in the prepupal period
it is known that the peak starting at about .817 millimeter and all higher
than this definitely refer to the last instar. But the other peaks occur so
irregularly that it is impossible to determine the number of instars between
the first and last.

UJ 72

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.254 .324 .409 .493 .578 .663 .747 .852 . .917 .959

WIDTH OF HEAD CAPSULE in mm.

Figure 4. The broken line represents the frequency distribution of the head
measurements of 1,816 larvae from a single tree. The solid line represents
2,598 larvae, the aggregate of those collected as single families. (From Kaston

and Riggsi)

It was thought that perhaps a sample from a more restricted and uni-
form environment would give a clearer picture. Accordingly, during the
second season (1935) 1,816 larvae from a single tree were measured.
The broken line in Figure 4 represents this series. There is, unfortunately,
a scarcity of younger larvae though the position of the first instar is suf-
ficiently clear. As in the preceding figure, the position of the last instar is
also clear, occurring in about the same place. Leaving out of consideration
the lower' instars. because of insufficient material, it is still impossible to
demarcate the penultimate and antepenultimate instars in which most of
the larvae fall.

Life History and Habits

During the third season (1936) it was decided to collect larvae by single
families and plot each family separately. The ideal situation to encounter
here is a family containing many larvae, with some in each instar. Figure 5
shows a histogram of a family of 75 larvae, one of the largest. Here can be
seen distinct groups indicating the presence of five instars. Many other
families showed a similar frequency distribution. On the other hand, there
were a number of families which, though not containing individuals from
all the instars, showed three or four groups of measurements arranged so
as to indicate a total of six instars. In these the instars I and II occurred
approximately as in the five-instar families. Moreover, the widths in the
sixth or last instar were about the same as those of the last instar in the
five-instar families. If all the families, totalling 2,598 larvae, are plotted
together, we get a curve represented by the solid line in Figure 4. There is
no doubt about instars I and II, but the remaining instars are difficult
to delimit. When the two sets of larvae are mixed, as they might be in
collections made at random, one might guess that there were possibly
eight instars, a number which never occurred when individual families
were plotted separately. This is a situation analogous to that which Gaines
and Campbell (13) found for the black cutworm, some of which have
six, and some seven instars. Of course, it does not follow that all the larvae
in any one family will have the same number of instars, and in fact evidence
was obtained from another source indicating that they vary. Our curves
can only show what the predominant number appears to be for each family.

>-
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WIDTH OF HEAD CAPSULE in mm.

Figure 5. Frequency distribution of the head measurements of a single family
of 75 larvae. (From Kaston and Riggs.)

By considering all the five-instar families, the mean head width for
each instar has been calculated. From these can be calculated a growth
ratio for each ecdysis, a mean ratio for all the ecdyses, and a set of theoreti-
cal mean head widths to test the possible application of Dyar's law. Prebble
and Bedard had found that this law could be applied to the species they
studied. Our findings are indicated in Table 2.

If one has only the first two instars from which to derive a ratio, the
theoretical means would be those given in the fifth column. If the mean of
all the ratios is used, the figures in the sixth column are obtained. As
would be expected, the latter fall nearer the actual means than the former.
Bedard had found for D. psendotsugae that the mean head width for 200

12 Connecticut Experiment Station Bulletin 420

Table 2. Comparison of Actual Mean Head Widths with the Theoretical
Figures Obtained by Using a Growth Ratio

Actual

Instar

Variation

Mean

Growth

Theoretical Mean

mm.

Ratio

Five- ins tar Families

// Ralio=1.322

If Ration 1.303

.254—324

.286 +.010

1.322

.286*

.324— .437

.375 +.013

1.343

.375*

.372

III

.423— .592

.506 ±.024

1.303

.500

.496

.564— .761

.646 +.029

1.242

.661

.632

.705— .931

.821 ±.031

.875

.824

Six-instar Families

If Rat

io-1.263

If Ratio^l.239

.268— .324

.287 ±.012

1.263

.287*

.324— .423

.363 ±.015

1.229

.363*

.356

III

.409— .507

.445 ±.017

1.274

.459

.441

.521—649

. 567 ± . 023

1.253

.579

.544

.634— .832

.711 ±.026

1.177

.731

.625

.747— .959

.837 ±.035

.922

.835

* Mean by actual measurement.

larvae in the first instar was .523 millimeter, and for 200 in the second,
.615 millimeter. The ratio between these is 1.175, and if one attempts to
determine the succeeding instars on the basis of this ratio one obtains
seven, with the mean width of the last 1.37 millimeters. This falls between
his observed actual mean of 1.38 and the theoretical 1.34. Yet he shows
definitely that there are only five instars in this species, so that use of the
ratio between the first two instars to determine the number succeeding
leads to error.

Becker (2) believes that five instars is the usual number for H. rufipes.
He considers that the irregularities in the peaks which appear when the
head widths of the later instars are plotted may possibly be explained by
assuming a variation in size of the two sexes. His measurements of the
heads of adult beetles indicate a difference of about 10 percent. Our own
figures based on 102 males and 115 females indicate the latter to be only
about 6 percent wider. But even a 10 percent difference, if such existed
in the larvae, would hardly account for the irregularities in the peaks.

To estimate the approximate duration of each larval stadium, Prebble
had taken the interval between the date when the particular instar formed
the majority of the population, and the date when the succeeding instar
formed a similar proportion of the population. Bedard, using Taylor's
method, counted the number of days from the first appearance of one
instar to the first appearance of the next and the number of days between

Life History and Habits 13

the last dates of appearance of the same two instars, added these two
numbers together and divided their sum by two. Our analyses of the
series of bead measurements according to dates of collection snowed that
practically all stages could be found in random samples at any time during
the season. Hence it is evident that neither of these methods could be
used successfully here.

In order to get more accurate information about larval activities we
prepared a device which enabled us actually to watch the larvae from
day to day. For the best results a piece of bark is selected which contains
a large number of eggs in a gallery, at least several inches removed from
other galleries. The outer bark is removed from a rectangular area of
about three by five inches, after which the external surface of the inner

Figure 6. Device for rearing larvae so as to keep them

under daily observation. Explanation in text. (From

Kaston and Riggs.)

bark is smoothed off as much as possible. It is essential that this be done
in order to have the piece of uniform thickness throughout. The piece is
now removed from the tree or log and immediately placed between two
plates of glass. The parent beetles, if present, should be removed first, and
care must be taken that none of the eggs (or young larvae) are displaced.
It requires considerable pressure to keep the plates close to the bark, and
thus prevent warping with the subsequent falling of the active larvae out
of their tunnels. The success of this rearing method depends in large
measure upon keeping the inner surface of the bark in intimate contact
with the glass. Elastic bands, as used by Bedard in a similar apparatus,
were not satisfactory, so we resorted to the device illustrated in Figure 6.
Four strips of wood, each about 7.5 inches long by 1.25 inches wide by .5

14 Connecticut Experiment Station Bulletin 420

inch thick, were prepared to serve as two pairs of clamps. Bolts were
placed about a half-inch from each end and the pieces of wood were beveled
so that only the center 2 inches of each was in actual contact with the glass.
In this way, on tightening the nuts, pressure was applied only directly
over the bark. It was found that the larvae got along best when the bark
was kept quite damp, so cotton was packed around the bark and moistened
daily. When not under actual observation, the entire device was wrapped
in black cloth or paper and kept at an approximately constant temperature
of 24° C.

Figure 7. Live larvae and their frass trails as
seen through the glass of the observation rearing
device, (x 6). The larva second from the left
has been underneath for some distance and the
place where its frass trail again strikes the sur-
face is plainly seen. (From Kaston and Riggs.)

With this apparatus it was possible to watch, for various lengths of
time, the progress of 31 families, including 374 larvae. The dates of hatch-
ing were obtained for 218 eggs, the dates of ecdysis accurately known in
552 instances, and estimated in about another hundred. Of the 50 larvae
that succeeded in pupating, the time of hatching was obtained for 28, so
that the duration of their entire larval life was known. Moreover, the
number of instars and duration of stadia are known from direct observation.

There is a very strong tendency for the larval tunnels to run wjth the
grain (Figure 7). Even when the egg gallery is not transverse, but inclined,

Life History and Habits

the larval tunnels, which start off at right angles to this, almost
immediately diverge from their original direction to become longitudinal.
However, there are instances where the larval tunnels, especially those
nearest the ends of the egg gallery, bend in a curve toward the latter.
In the case of a female completing one arm of a V-shaped biramous gallery
before laying any eggs in the other arm, the larvae hatching later almost
always eventually encounter the frass trails of older members of the family.
If, as Tragardh (33) believes, the larva is able to detect the degree of
decomposition or dryness of the surrounding bark and thus be guided in
the proper direction, there is the apparently aberrant behavior of occasional
larvae suddenly changing direction and crossing the frass trails of neighbor-
ing larvae, or even proceeding back toward the egg gallery in their own
or another's frass trail! Yet a larva never failed to change its direction
whenever it approached the cut edge of the piece of bark.

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TIME IN DAYS

Figuke 8. Progression curves of three different larvae, as well as of the mean

determined for three families. The "p" indicates the place at which pupation

occurred. (From Kaston and Riggs.)

By measuring the length of its tunnel each day, a curve can be plotted
for each larva, such as is shown for three individuals in Figure 8. It was
found that accompanying each ecdysis there is a period of almost a day
during which the larva does not feed. This is represented by a short
horizontal line along the curve. Also obtainable from these curves are the
total duration of larval life, the duration of each stadium, and the number
of stadia. For example, larva N-l had seven stadia, the duration of which
and the distance progressed during each being as follows:

Instar I, 4 days, 2 . 5 mm.

II, 5 days, 5.3 mm.

III, 4 days, 5 mm.

IV, 6 days, 6.1mm.

Instar V, 7 days, 9 . 3 mm.

VI, 7 days, 11.4 mm.

VII, 11 days, 10 mm.

16 Connecticut Experiment Station Bulletin 420

It did not tunnel for about three and a half days before pupation, and
remained eight days in the pupal stage. In like manner, larva S-17 went
through six instars in 36 days, tunneling a distance of 60.8 millimeters.
Between the twenty-ninth and thirty-third days it was not visible (indi-
cated by the dotted line in the curve), having gone underneath the surface
of the bark. It was later dug out on the day of pupation at a point 8 milli-
meters beyond the point at which it had disappeared.

The larvae of families N and 0 taken together present a curve showing
a much slower rate of tunneling than those of family S. These curves
also show what has been found true in general for the other families as
well, namely that there is a positive correlation between the number of
instars and the duration of the larval period. Larvae have completed
their development in 6 to 12 instars, and in Table 3 is indicated the mean
duration of the larval period.

Table 3. Mean Duration of Larval Period

Number of instars

Duration

Number of larvae

The instances of 9 to 12 instars all occurred in family A, which devel-
oped under abnormal conditions. It was one of the first families reared,
and the bark was not kept tightly against the glass at all times. This re-
sulted in drying of the bark alternating with flooding when an attempt
was made to remedy the situation. This shows, however, that the larvae
are potentially capable of molting more than the normal number of times.
Some of the irregularities in our curves (Figures 3 and 4) may be due to
retarded larvae which are going through a greater number of instars.
In this connection it is of interest to note that Metcalfe (26) was unable
to arrive at any satisfactory conclusion concerning the number of instars
in the anobiid, Sitodrepa panicea L. The curve of head-capsule measure-
ments plotted from random collections presented nine peaks which the
author hesitated to accept as representing a like number of instars, "a
number hitherto unprecedented in the Coleoptera." Instead, evidence was
presented to indicate that four of the peaks belonged to one sex and
five to the other.

There seems to be no correlation between the distance tunneled and
the duration of larval life. However, the mean of the distance traveled
increased in successive instars, as is to be expected.

Life History and Habits

Different larvae showed great variation in their rate of tunneling, as
shown in Figure 9. Here the slope was obtained from curves similar to
Figure 8 simply by connecting the point representing the time of hatching
with the point where the larvae stopped tunneling in preparation for
pupation. For larvae A-4 and A-10 these curves extend to the time of
pupation. Larva N-2 went underneath the bark surface during the seventh
stadium so that the dotted line was continued from this point to the day
when the average for this family pupated. Larva Q-d was lost to view
during the sixth stadium, the dotted line indicating the probable course to
pupation, the average duration of larval life not being known for family Q.
It can be readily seen that larva A-10 had traveled about 13 millimeters in
the first 20 days, whereas larva N-2 had gone just about twice that far, and
. larva Q-d about three times.

/
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E
E 40

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Q
Ll)

Ul 30

(£

// ^

/ /

^ 20

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/i /

/ / /

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TIME IN DAYS

Figure 9. Rate of tunneling -of various larvae. Explanation in text. (From

Kaston and Riggs.)

After molting, and when the new head capsule has hardened, the larva
turns around in its gallery and eats the exuviae. Blackman suggested
that the larvae of P. hopkinsi have the same habit because he did not always
find their exuviae in the frass trails. Prebble had likewise encountered
similar difficulty with the three species previously referred to. However,
we have found that only the relatively soft body cuticula is eaten, leaving
the head capsule. This gets broken up so that only certain parts, especially

18 Connecticut Experiment Station Bulletin 420

the mandibles, may be distinguished later. At intervals the larva turns
around and packs the newly voided frass with its head, using its mandibles
to tamp it down firmly. During this process the remains of the capsule
often become buried or obscured. With material brought in from the field
we have found it possible, by careful manipulation under a binocular, to
expose fragments at intervals in the tunnels. The discovery by this method
of five groups of capsule fragments in a few tunnels indicated to us that
there could be at least six instars. Since the fragments are pushed about
in the frass trails, it is not possible to deduce from their spacing how far
the larva travels during each stadium.

Pupal Period

Before pupation the larva in the last instar spends about a day and a
half during which it does not progress farther, but simply enlarges the end
of its tunnel to form a pupal cell. This may be called the ante-prepupal
period, and is followed by a prepupal period during which the larva is
quiescent for almost two days. It shortens slightly, loses some of its curva-
ture, becomes creamy white, and thickens around the thorax.

The pupal cells are almost always built in the inner bark alone. They
are seldom in contact with the wood surface, more often touching the
outer bark. Very rarely, especially when the log is quite dry, the pupal
cell may be entirely in the outer bark. The oval cell is built with its long
axis parallel to the grain of the bark (Figure 1).

In the constant temperature room (about 24.5° C. and 65 percent
humidity) the average duration of the pupal period is 7.26 days. On
the first day, the pupa is entirely white. On the second, there is a faint
suggestion of pigment in the eyes. On the third, the eyes are faintly tan.
On the fourth, they are brown to red ; on the fifth, dark brown to black ;
on the sixth, the mandibles acquire a brownish-red color, and the wing
tips become gray. On the seventh, the legs, head, and pronotum become
orange. Emergence from the pupal skin takes place while most of the
body is still white. After emergence the callow adult colors up rapidly
before beginning to cut its way out of the pupal cell. The pupal stage
under field conditions in July is estimated to be eight or nine days in length.

Table 4. Duration of Pupal Stage at Various Temperatures

Temperature
degrees C.

Number
of pupae

Number
emerged

Percent
emerged

Number of
days pupation

3.2

57. 7

23.3

20.5

10.94

24.5

7.26

6.81

5.42

Life History and Habits

Since the pupal stage is quiescent, it is a relatively easy matter to test
the effects of different temperatures on the rate of development. The
prepupae were removed to dishes and placed in incubators at various
temperatures. The figures in Table 4 indicate the results obtained, and
the results are shown graphically in Figure 10.

DAYS

10 20

DEGREES CENTIGRADE

Figure 10. Effect of temperature on the development of pupae. The
solid lines follow the actual data. The broken lines indicate the following
theoretical curves: hyperbolic: K = Time x (Temp. -8.8); rectilinear:

122_ = .9 Temp. -7.9.
Time

20 Connecticut Experiment Station Bulletin 420

The time- temperature relations of insect development have been
expressed frequently by the formula K = TimeX (Temp. — Z), where K
is a constant and Z is the "physiological zero". This is the formula for an
equilateral hyperbola. Although it probably does not represent the true
conditions at the upper and lower temperature limits at which develop-
ment can occur, nevertheless it does represent the effect of temperature on
insect development within certain limits. The data given in Table 4 fit
rather well the formula K=Time X (Temp. — 8.8), and the hyperbolic
curve in Figure 10 represents this formula. The greatest divergence of the
theoretical from the actual is at the 10° level. At this temperature rela-
tively few pupae survived, and it is so close to the "physiological zero"
that slight divergences are accentuated.

The reciprocal curve, where the temperature is plotted against the
reciprocal of time, illustrates the effect of temperature on the rate of
development. This curve is rectilinear and is also shown in Figure 10.
The data correspond rather well to the formula -122. — 9 Temp —7 9 (the

Time

reciprocal of time is multiplied by 100 to avoid decimals). The curve has
been projected to meet the X axis at the theoretical "physiological zero",
8.8° C.

It appears that pupal development will not take place at temperatures
below about 8.8° C., but such development will certainly occur at tem-
peratures exceeding 30° C. Between 10° C. and 30° C. the rate of devel-
opment is directly correlated with temperature, and the relation of increase
in developmental rate to increase in temperature may be expressed by a
rectilinear equation. Pupae held at 5° C. for 60 days showed no signs of
development, no pigment appearing even in the eyes. When removed to
24.5° C, 11 of the 64 specimens emerged normally in 7 days; the others
died. The mean number of days taken for pupae held continuously at
24.5° C. was 7.26. This supports the contention that the so-called
"physiological zero" for pupae is at about 8.8° C. (about 48° F.).

Emergence of Adults from Bark

After the callow adult has hardened somewhat, it gnaws its way to the
outside. The exit tunnel is ordinarily cut at right angles to the surface
and the beetle emerges through a "shot" hole of about 1.25 millimeters
diameter (Figure 1). In some cases, where the outer bark is somewhat
loosened from the inner, the beetles cut through to the outer surface of
the inner bark and groove this surface between the two bark layers until
they can emerge through a crack, or at the broken end of the log, etc.
Selecting a random sample of several hundred, the sexes were found to be
about equal in numbers.

By placing logs in cages and checking regularly, one can count the
number of adults that emerge each day, and it is possible to determine
the time of the peak emergence. In the accompanying graph, Figure 11,
the emergence curve for log number 373 is shown. This log was enclosed
with about 275 beetles and kept in a constant temperature room at about
24.5° C. during the entire period of development. Young beetles emerged
over a period of 52 days of which the first occurred 57 days after the
log was exposed to attack. The peak of emergence occurred at about

Life History and Habits

74-75 days, or 17-18 days after the first beetle emerged. The shape of the
curve follows that of a normal curve, superimposed upon it in the graph,
but there is a tendency for late emergences to be prolonged for a consid-
erable period.

W 100

8 7

DAYS

Figure 11. The solid line represents the curve of emergence of young adult
beetles (which were kept at a uniform temperature for the entire period of their
development) beginning at 57 days after the log was subjected to attack. The
approximation of this to a "normal" curve is indicated by including the latter

as the broken line.

Emergences from logs in nature are usually spread over a period of two
months or more, and during the peak the number each day is roughly cor-
related with the temperature. Figure 12 indicates this correlation (for a
portion of the emergent period) in the case of two logs caged in an outdoor
insectary. The first beetle emerged August 2, though the peak of the
emergence is about the third week in August. Somewhat similar results
were obtained with other logs in previous seasons, a small emergence
starting just after the middle of July and the peak occurring in either the
second or third week in August. This, of course, refers to progeny from
spring-laid eggs.

It is difficult to determine just how many beetles come from any one
family. The egg galleries are usually placed so closely that the larval
tunnels and exit holes of neighboring families intermingle. By the time
the brood emerges the larval tunnels are not distinct and cannot be counted.
It then becomes necessary to obtain an average by counting exit holes,
removing the bark and counting the egg galleries. These figures are
supplied in Table 5.

Connecticut Experiment Station

Bulletin 420

The logs selected were those which appeared to have "normal" galleries
and which had not dried too much. Of 14 logs examined after the broods
had emerged the highest average was 21.5 young beetles per family, and
the lowest was 11.9. The mean of these averages is 15.6. Since, as already
stated, the average number of eggs laid per female is about 60, this indi-
cated that the hazards of life under field conditions from egg to young
adult reduce the population 75 percent. A reduction of 85 percent was
reported for Dendroctonus brevicornis Leconte by Keen and Furniss (23).

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DATE

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7 II 15 19

September

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Figure 12. The solid line represents the number of young adult beetles emerging.
The broken line indicates the average daily temperature.

Occasionally, when a piece of bark is removed, it is found to contain a
family in which the larval tunnels are not confused with those of neighbor-
ing families. From one such, of which the egg gallery was 20 millimeters
long, there emerged 27 young beetles; and in another, 11 millimeters long,
there were 14 exit holes.

Flight and Wind Carriage

To obtain information on the flight powers of the beetle we set up trap
logs in two elm-free areas. The first of these was a sand bar projecting out
from the mainland proper at Old Lyme. Freshly cut logs were suspended
from tripod arrangements so as not to touch the sand, and shade was
provided for each log. The logs were placed at approximately 300-yard
intervals extending south and west of the nearest elms. On three occasions
a beetle was collected at a log three-fourths of a mile out.

The second of these elm-free areas was Charles Island, approximately
three-quarters of a mile off the Milford coast. The nearest elms are. about

Life History and Habits

one-fourth mile back from the coast. Two beetles were recovered from one
log and one from another. Hence they must have flown about a mile,
though it is possible, of course, that they were blown by the wind. Felt
has shown in a series of papers (9, 10, 11) that even heavy-bodied insects
like beetles can be distributed by the wind. His experiments with balloons
also indicate that the prevailing winds in this region are northeast. There
is thus the further possibility that the beetles taken from the logs in our
elm-free areas may have come from farther away than the nearest elms, for
the latter were north and east of the trap logs.

Table 5. Adults Emerging per Family from Logs

Location

Log
number

Number of
families

Number of
exit holes

Average number

young beetles

per family

Riverton

148

1764

11.9

Riverton

1603

17.8

Riverton

456

18.3

Riverton

120
271

140
166

1882
3562

13.5

West Hartford

21.5

Orange

321

106

1685

15.9

Orange

320

130

1933

14.9

Franklin

378

117

2086

17.8

Franklin

379

124

1816

14.6

New Haven. .

351
353

42
41

515
652

12.2

New Haven

15.9

New Haven

373

1536

16.7

Cheshire

380

177

2387

13.5

Cheshire

383

125

1863

14.9

It is probable that emerging beetles will not attempt to fly long dis-
tances, but rather are attracted to the nearest satisfactory elm material.
This is indicated by the fact that bark tunnels in healthy trees are more
numerous the nearer the trees are to logs from which beetles have emerged.

Bark Tunnels in Living Elms

Within 24 hours of their emergence from the bark, the callow adults
may enter logs to breed, as indicated by their behavior in the insectary, but
we have found that large numbers of them will first fly to nearby healthy
elms and feed there. Experiments during the 1935 season showed that
when a number of beetles were enclosed in a cage containing only freshly
cut twigs, they made feeding tunnels similar to those known to be made by

Connecticut Experiment Station

Bulletin 420

Scolylus multistriatus, but when freshly cut logs of three- to six-inches
diameter were also provided, they tended to attack the latter rather than
the twigs. Accumulations of frass in crevices of the outer bark may lead
one to suspect that the beetles have burrowed in to build egg galleries.
But if such a place is investigated thoroughly, it will be found that instead
of an entrance hole leading to the wood surface, there is a shallow tunnel
more or less parallel to the surface, just under the outer bark (Figure 13).

Figure 13. Surface view of a portion of trunk show-
ing bark tunnels natural size. (From Kaston and Riggs.)

Those beetles which develop from overwintering larvae will be found
making such bark tunnels in early to middle June. But those which
emerge from eggs laid in the spring and early summer will be found in
largest numbers in August and September. Many of these latter will not
start brood galleries until the following year, but will hibernate in the
bark tunnels on the healthy trees (Kaston and Riggs, 22).

In 1937 the first bark tunnels were observed in the field on July 22,
which was shortly after the young beetles had started emerging. From
July 30 to September 3 live elms were examined at intervals of three or
four days. During this period notes were taken on a total of 291 bark

Life History and Habits 25

tunnels in the tops, trunks, and exposed roots. The average length was
3.4 millimeters, and more than 90 percent lay in part in the inner bark. Some
approached quite close to the surface of the wood (Figure 14), and many
contained droplets of sap, but none attained the sapwood as reported by
Becker (1). While some of the tunnels had been deserted by the beetles,
51.5 percent were still occupied. The direction of tunneling varied, with
45 percent up, 11.3 percent down, 5.5 percent transverse, and 34.3 percent
oblique. In 3.8 percent the direction could not be determined, as apparently
the beetles had not progressed far enough. The ratio of occupied to de-
serted tunnels for this particular group is rather high. On September 15,
of 150 tunnels examined only 38 were occupied, or 25.3 percent. Many
random samples gave even lower ratios.

I v. ;fe

\ i V J

H I I I 1 1

J V

•if.- '

I *

Inner bork j, Ouler bor
l if

•■■M I"

Figure 14. Semi-schematic drawing of a longitudinal section through a bark

tunnel, showing the relation to outer and inner bark layers, and to wood (about

four times natural size). (From Kaston and Riggs.)

Though the bark tunnels are found most commonly on the trunk, they
occur from the ground level all the way into the smaller limbs. Figure 15
shows a few in a one-inch branch, and we have seen them in a five-eighths-
inch branch. To obtain some idea of their density at various levels on the
trunk and leaders, a healthy tree was selected at random from among
those standing in an area where beetles had emerged. The tree was quite
straight, had a diameter breast high of 10 inches, and presented a fairly
even surface to about 48 feet from the ground. Five levels were chosen at
intervals of approximately 10 feet. At each level a section of bark 18
inches in height was carefully gone over, and only those tunnels noted
which gave evidence of being recently made, i.e., not from the previous
years.

The results of the observations from September 6 to 9 inclusive are
shown in Table 6. If these areas sampled are any indication, there must
have been a very large number of beetles in the entire tree. The estimate
given for the trunk includes only one of the main leaders. The density of
these tunnels is greatest in the region between 12 and 33 feet above the

Connecticut Experiment Station

Bulletin 420

ground. The percentage of occupied tunnels is lowest in this same region.
This would indicate that this part of the tree is preferred by the first
beetles to emerge from breeding places. The upper and lower regions are
attacked by the later emerging beetles, after some of the early ones have
left to find suitable breeding places.

Figure 15. Small branch with bark tunnels,
natural size. (From Kaston and Riggs.)

Table 6. Bark Tunnels in a Healthy Elm Tree at Various Levels
Above the Ground

Diameter

centimeters

Bark tunnels present

Total
number

Density per
square decimeter

Occupied

Number

Percent

2 ft. 6 in. to 4 ft.

26.8

1.3

30.6

12 ft. 6 in. to 11 ft.

20.4

423

/
14.4

11.8

22 ft. 6 in. to 24 ft.

14.1

266

13.8

10.9

31ft. 10 in. to 33 ft. 4 in.

232

13.7

9.5

42 ft. 2 in. to 43 ft. 8 in.

6.2

6.6

17.2

Whole trunk

(Estimated)

Bottom 30
Top 5

6753

8.3

818

12.1

Chapman (5) was the first to have noted that the adult beetles may
overwinter, and that this hibernation takes place in healthy trees was
pointed out by Becker (1). From our observations at regular intervals from
July through the succeeding fall and winter, we are inclined to believe
that there is no fundamental difference between hibernating tunnels and
feeding tunnels. The beetles emerging early make their bark tunnels and
then leave to construct egg galleries elsewhere. Those emerging later may
construct bark tunnels late enough in the season to find their activity
considerably retarded by the low temperatures prevailing during constantly

Life History and Habits

increasing portions of each day. Hence they do not leave to construct
brood galleries, and their feeding tunnels now become their hibernating
tunnels. The latter are, in general, longer than the former. It has also been
observed that the ratio of unoccupied tunnels in the smaller branches,
where the bark is thin, increases during October and early November.
This is believed to indicate that some of the beetles had left to seek thicker
bark in which to hibernate. Young adults which emerged as late as Novem-
ber 6 apparently made hibernating tunnels in thick bark without first
feeding in thin.

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HEIGHT ABOVE GROUND IN FEET

Figure 16. Curves showing the relation of the level at which beetles occur in bark
tunnels to the density, and to winter mortality, etc.

Becker (1) reported late September as the time when first signs of
hibernation were noticed, but he was then unaware that bark tunnels could
be made earlier in the same places. To determine whether beetles were
still active and feeding, the alimentary canal was dissected out and the
ventriculus examined for food1. Although some beetles were found already
inactive and with empty ventriculi on October 3, we found many still
active and feeding right through to the end of October. It is evident that
the prevailing local temperatures influence their activity oh any given day,
and that some of the beetles could have backed out of tunnels in September
or earlier to construct new ones in which to hibernate.

1 It may be mentioned parenthetically that experiments similar to those carried on with the larvae
(p. 9) indicated a lack of ability to digest cellulose.

Connecticut Experiment Station

Bulletin 420

Hibernation

As discussed above, beetles may winter over as adults in bark tunnels
made in live trees. Comparable areas of bark at various levels on the same
tree used in the previous study were examined for hibernators about
February 26, 1938. The beetles were carried into the laboratory and dis-
sected to determine whether they were alive or dead. The results are
shown in Table 7 and indicated graphically in Figure 16. The density
and percentage of survival are greatest in the lower part of the tree where
the bark is thick, and lowest in the upper part where it is thin. Taking
the 137 beetles together, 56 were dead, a mortality of 41 percent. Of 116
adults collected at random from other localities, there were 71 dead, or 61
percent. Of 163 collected the previous season, 49 were dead, or 30 percent.
The sum of these is 176, or 42 percent dead out of a total of 416 beetles.
m ■

Table 7. Hibernating Beetles in a Healthy Tree at Various Levels Above

Ground

Diameter

centimeters

Beetles

Height above Ground

Total*

Density
per square
decimeter

Live

Dead

Mortality
percent

4 ft. to 5 ft. 6 in.....-..,

14 ft. to 15 ft 6 in

24 ft. to 25 ft. 6 in

30 ft. 4 in. to 31 ft. 10 in.
41 ft. 10 in. to 43 ft. 4 in.

24.8
19.8
12.7
11.5
5.1

38
23

22
2

1.82
1.36
1.27
1.35
.28

7
0

32.2
14.7
75.0
68.2
100.0

* This includes the few accidentally lost in the collecting process.

In addition to their hibernating in bark tunnels of live trees, in rare
instances they may be found elsewhere. Ordinarily tenerals which are
unable to emerge from pupal cells die sometime during the winter. How-
ever, a live teneral specimen was taken from a pupal cell at East River.
March 26, 1936. Four were found in old egg galleries as follows: one at
Mount Carmel, March 24, 1936; another at Groton, April 26, 1935; a
third at Washington, April 1, 1936; and a fourth at Branford, March 13,
1938. The circumstances concerning the latter are particularly interesting.
This tree had a diameter of 12 inches at breast height. It was among
those examined on November 6, 1937, which at that time were partly dead.
On the trunk were many beetles in bark tunnels, and also some in entrance
holes leading to the wood surface. A few very short egg galleries could be
found, but no eggs. On March 13, 1938, only one egg gallery had a beetle
in it, but there were several beetles found in bark tunnels which were
directly connected by the entrance tunnel to the gallery beneath. Several
of the trees examined in the same area had similar connections of bark
tunnels with egg galleries. The females found in these bark tunnels in

Life History and Habits 29

association with the egg galleries all had spermatozoa in the seminal
receptacles in March. It seems likely, therefore, that the beetles started
breeding tunnels late in the fall and then moved out into bark tunnels for
the winter. We believe this to occur only very rarely, when a tree is in
healthy enough condition in the fall to attract bark tunnel makers and
then soon becomes receptive to them for breeding.

In the few cases in which eggs were found as late as November 28, we
tested their viability by transferring 25 to the laboratory constant tempera-
ture and humidity room. All hatched out to normal larvae. All of the eggs
found in similar galleries in these trees on March 13 must have been dead,
for of the 42 placed in the constant temperature room, none hatched.

As is to be expected, the appearance of genitalia during hibernation
differs from that during the breeding season. In the hibernating males the
testes and seminal vesicles are more compact. Hibernating females have
the ovarioles thin and the colleterial glands small.

Since, of course, the larvae hibernate in the bark of the dead limb in
which the beetles have been breeding, they are much easier to find in
quantity. All instars can be found but the older ones are more common.
The larvae may continue to feed during the warmer parts of the day until
quite late in November but slowly become more and more white as the
food disappears from the intestinal tract.

Counts were made at intervals to determine the winter mortality. For
the 1936-37 season, from a total of 2,190 larvae only 101 were dead, or
5 percent. In 1937-38, 38 were dead from a total of 1,656, or 2 percent.

Pupae are found only rarely during the winter. It would appear that
ordinarily the lower temperatures of late fall prevent transformation to the
pupal stage even when they do not prevent larval activity. Several logs
which contained old larvae near pupation were placed in the insectary for
observation. Of 22 pupae which were found in the late winter, 4 were
dead; i.e., a mortality of 18 percent. Of 55 pupae found outside, pre-
sumably lacking the partial protection of the insectary, there were 16
dead, or 29 percent.

Number of Generations

The number of generations and the time when the different stages are
present throughout the year can be seen from the diagram, Figure 17.
The dates of beginning activity naturally vary according to the tempera-
ture. Generally speaking, hibernating larvae begin to feed about the time
the elm flowers appear. Some idea of the amount of variation may be
gained from the following:

1935 1936 1937 1938

Flowers began to appear .... April 3 March 26 April March 23

Leaves began to appear .... April 25 April 20 May 1 April 15

The height of the peaks in Figure 17 does not represent the number of
individuals for any given stage. The interval indicated between the first
appearance of one stage and the earliest date for the succeeding stage is
based upon our calculations of the duration of the stages according to the

Connecticut Experiment Station

Bulletin 420

average temperature at the particular time of year. Figures from the
Weather Bureau office indicate the following average temperatures for
New Haven:

May 57.9° F.

June 66.6°

July. ....... 71.8°

August 70.3° F.

September.. 63.7°

We proceeded on the assumption that the effect of temperature on develop-
mental rates in the other stages was similar to that in the pupal stage. Al-
though this assumption may be questioned, the results of the calculations
checked well with field observations.

Winter

April

May

June

July

Aug.

Sept.

Oct

Nov.

Winter

April

May

June

July

Aug.

Sept.

Oct.

Nov.

Winter

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Figure 17. Diagram of the life cycle through two consecutive years. Solid black
represents Generation A; vertical lines, Generation B; stippfed areas, Generation C;
and slanting lines, Generation D. The pupae of Generation A give rise to the adults

of Generation B, and so on.

Starting with the A Generation adults, we find them hibernating
during the first winter under consideration here. About the end of April
the earliest ones start entering breeding material, and the latest will finish
egg laying about the middle of July. The first eggs of this generation appear
about the beginning of May, the first larva about the middle of May, and
the first pupa during the second week of July. While most of these larvae
will develop to pupae before late autumn, a few will be retarded and pass
the winter in the larval stage. This represents a single generation per
year. A still smaller number of pupae may overwinter. Ordinarily the
first adults of the new (or B) Generation, from eggs laid by A, may be
found as tenerals beginning about the middle of July. Most of the adults
of this brood produce the eggs found in late August and September. Those
emerging late may not enter to breed, but may be found in bark tunnels
through the autumn and hibernate there during the winter. The eggs of

Natural Factors of Control 31

the B Generation give rise to the larvae which we believe to form the bulk
of the hibernating population.1 Eggs laid late in the fall (after the latter
half of October) , die during the winter.

The first tenerals of the C Generation, from hibernating larvae, appear
toward the end of May and emerge from the bark about the middle of June.
Their main oviposition period is in July and August. A few of the larvae
from the eggs hatching late in the summer will winter over as such. The
greatest number will give rise to the D Generation adults which winter
over in bark tunnels, but a few of the adults will start brood tunnels.
There is thus the possibility of having two generations during one year, for
some of the eggs hatch before the onset of killing cold, as in the case of the
B Generation. For the somewhat warmer climate of New Jersey, two gen-
erations per year appear to be more usual, according to Collins (6).
Martin (25) believes one generation to be usual where he worked at Patter-
son, N.Y., but that locality hardly differs in topography and climate from
the nearby Connecticut towns of New Fairfield and Danbury. We believe
he has erred in his interpretation partly because he has restricted his
observations to trap logs only.

NATURAL FACTORS OF CONTROL
Temperature

The factors involved in the control of this insect have not been thor-
oughly investigated. The climate of Connecticut is such that it plays
very little part as a limiting factor. That severe winter temperatures may
kill a large percentage of an overwintering bark beetle brood has been
shown by Keen and Furniss (23). To gain some idea of the effects of low
temperatures on H. rufipes several experiments were conducted, chiefly
with the larvae, in which stage we believe by far the largest proportion of
the population spends the winter. The method followed was a modifi-
cation of the thermocouple technique given by Robinson (29, 30).

The thermoj unction was placed in contact with the body of the speci-
men which lay at the bottom of a tube. The whole tube was immersed in
a Dewar flask containing the cooling agent and the readings were made on
a previously calibrated galvanometer scale. The temperature was lowered
slowly and was allowed to drop beyond the freezing point to the super-
cooling point, as indicated by a deflection of the galvanometer due to the
liberation of heat of crystalization. The temperature rises to the true
freezing point, which can be read when the galvanometer makes a tem-
porary halt before continuing down again. In Table 8 are given the results
obtained with hibernating larvae. It is of interest to note that when
these larvae were removed from the apparatus after being frozen, and
allowed to attain room temperature, they all survived. Similar experi-
ments with larvae that had already begun spring feeding showed a lessened
resistance to the low temperatures. Furthermore, none survived the
effects of the freezing. A third set of experiments run with pupae, which
had just transformed from overwintering last instar larvae, surprisingly
enough gave readings rather close to those of the latter. However, all
the pupae died.

1 Martin (25) , in a paper which has j ust appeared, states that 80 to 90 percent of the population winters
over in the adult stage. No evidence is given in support of this statement, which is contrary to our own ob-
servations. Nor is any evidence supplied to substantiate his statement that some larvae go through a "dia-
pause" condition for more than 16 months so that they spend two winters in this stage. This would seem
to require further explanation.

Connecticut Experiment Station

Bulletin 420

Table 8. Results of Low Temperature Experiments. An Analysis of the

Differences Among the Three Groups Listed Shows them to be

Statistically Significant

Temperature degrees C.

Number

of
specimens

Highest

Lowest

Mean

Larvae :

Overwintering -supercooling.. . .

-18.7°

-28.0°

-24.4°+ .2°

-freezing

-10.0°

-23.5°

-16. 4° +.4°

Feeding -supercooling.. . .

- 6.3°

-20.3°

- 9. 7° +.5°

-freezing

- 2.3°

-12.3°

- 4.7°+. 3°

Pupae: -supercooling.. . .

-14.5°

-23.3°

-21. 7° +.2°

-freezing

- 7.8°

-18.6°

-13.0° ±.3°

Although winter air temperatures in Connecticut sometimes do get
down to — 24.4° C, equal to about — 10° F., it must be remembered that
the bark offers some protection. Not many overwintering larvae would be
frozen. On the other hand it is possible for some larvae to die as a result
of a cold spell coming on after they start feeding. The figures do not
explain the scarcity of pupae during the winter, but they might in part
account for the high mortality of this stage.

Moisture

The fact that this species rarely enters on the upper side of a horizontal
limb in sunlight would indicate that it is intolerant of the higher tempera-
tures on that side. Of course, there is the factor of dryness to be consid-
ered, and we have already pointed out (page 5) that a loss of more than
half the original water content will render the logs unreceptive to attack.
Once a brood is started, quick drying of a small limb, i.e., about two
inches in diameter, often kills the larvae before development can be
completed.

Competition

There are instances where the density of egg galleries is so great that
the larval tunnels coming off from them become confused while the larvae
are still quite young. Many of these larvae are considerably retarded in
their development, and no doubt many die. Besides overcrowding by
individuals of their own species, many larvae lose out in competition
with that most abundant of elm bark insects, Saperda tridentata, the larvae
of which often ruin whole families of H. rufipes.

Predators

Hopkins (14) lists Thanasimus dubius Fabricius as a predator of H.
rufipes, but the commonest predator in our experience is Enoclerus

Natural Factors of Control 33

nigripes Say (quadriguttalus auct.). Adults of E. nigripes can be found
running about over the bark of elms most commonly in late May and June.
They are voracious feeders, one specimen in the laboratory consuming
five adults of H. rufipes in succession. They usually begin by disarticu-
lating the head from the body, proceed from the head to the thorax, and
end by cleaning out the abdomen, leaving only the cuticular structures.
The entire process takes from 7 to 20 minutes.

Mating of E. nigripes probably occurs from the middle of May to the
end of June, and a pair was observed in copula at Orange, June 17, 1935.
Eggs can be found in June in the galleries of H. rufipes. On June 16, 1937,
there were four E. nigripes eggs lying in the egg tunne! of an H. rufipes
gallery. The latter had 20 larvae, none older than the second instar. The
larvae of the predator feed on the bark beetle larvae to about the middle of
August. They then migrate to the outer bark and construct a pupal cell.
Emergence as adults occurs after the middle of September and the beetle
hibernates in the adult stage. Specimens pupating in early September
and placed in the constant temperature room at about 24° C. emerged in
12 or 13 days. The absence of parent bark beetles, especially the male,
from egg galleries with eggs or young larvae can usually be attributed to
their having been eaten by this predator.

While we have not actually observed the feeding of Platysoma coarc-
tatum Leconte, it is reasonable to suppose from analogy with P. punctigerum
Leconte, reported by Struble (32) to be a predator of various scolytids,
that this species is also a predator. It is fairly common in bark containing
old galleries of H. rufipes. Eggs were found in June in the larval tunnels.
Pupation occurs in late August, not as inE. nigripes in a special pupal cell in
the outer bark, but in the egg galleries of its prey.

Another, perhaps facultative, predator is the fly, Lonchaea polita Say,-
which is sometimes found in numbers. This also pupates in the egg galleries
of H. rufipes.

Parasites

The commonest parasite encountered is the braconid, Spathius canaden-
sis Ashmead (Figures 18 and 19). A list of the other hosts of this parasite
has been published (Kaston and Becker, 20) and some nctes on its habits
have been recorded by the present author (18). This parasite has been
observed to vary considerably in its abundance in different years. It seemed
particularly abundant during 1935 and 1936, but has been noted only
occasionally since then.

It has been found that beetle larvae of the penultimate and ante-
penultimate instars are attacked by Spathius, but by far the largest num-
bers are attacked in the last instar. Emerging adults vary considerably
in size, and it is suggested that this may be due to the fact that develop-
ment can evidently be completed on host larvae of different ages. The
parasite attaches itself to the side or dorsum of the host (Figure 19 D),
often with its head at the level of the host's metathorax.

This species hibernates as a prepupa in the cocoon, but, as this stage is
also found in July, and young adults may emerge in early August, it would

34 Connecticut Experiment Station Bulletin 420

appear that there are two broods per year. Exact data on the complete
life cycle are lacking.

The parasitized host becomes entirely motionless and white in color,
and, even though the parasite on it is quite small and inconspicuous, can
easily be distinguished from unparasitized larvae. The cocoon of the

Figure 18. Spathius canadensis. Dorsal aspect of female,
enlarged about 12 times. (From Kaston.)

parasite is pale yellow in color when first made, later turning to a dark
brown. It is papery, covered with loose fibers, and usually has adhering
to it particles of frass from the host's tunnel. The average dimensions of
17 cocoons were 4.5 by 1.4 millimeters. The pupal period is nine days at
24° C. The imago emerges from the bark through a circular hole about
.75 millimeter in diameter, readily distinguished from the exit holes of its
host which are about 1.25 millimeters in diameter.

The number of parasitized larvae seldom exceeds 25 percent in any
one family, though in one instance we noted three-fourths of the larvae

Natural Factors of Control

with parasites. Usually the number of parasites is nearer 5 to 10 percent,
and that only in certain localities, so that it may not play a role of great
importance in the natural control of H. rufipes.

A parasite even less common and hence about which practically nothing
is known was found to hibernate as a prepupa within the bodies of the
overwintering adult beetles. This has been determined by A. B. Gahan as
a new species and new genus of Pteromalidae. Each parasitized beetle
has but a single parasite, which pupates without building a cocoon. The
period of pupation is about 10 days at 24° C.

Figure 19. Spathius canadensis. A, pupa. B, cocoon. C, anterodorsal
aspect of the prepupal head. D, feeding larva on its host. E, lateral aspect
of the abdomen of an adult male, and F, of an adult female. (From Kaston.)

Nematodes have been reported from various Scolytidae. While Steiner
(31) supposes that they may destroy beetles, he gives no data. Oldham (27)
points out that the presence of nemas in the body cavity can have the
effect of reducing the size of the gonads and even lead to sterility.

Approximately four-fifths of the adult beetles examined from mixed
localities had nemas in the coelomic cavity. They were as often found in
one sex as the other. In only a very few was there any diminution in the
size of the gonads, which might be attributed to the nemas. Of the many
larvae and pupae examined none were found to contain nemas. One batch
of prepupae was removed to dishes and obliged to pupate and emerge as
adults free of all bark. After the tenerals had hardened somewhat they
were dissected and examined. Of 44 examined 36 had nemas. It is obvious,

36 Connecticut Experiment Station Bulletin 420

therefore, that the nemas must be present in the larvae and pupae, but in
some minute embryonic or larval form not readily seen, and only develop
when the beetles reach the adult stage.

Mites are often found in the galleries of H. rufipes and a number of
species are carried about on the bodies of the beetles. The significance
of these as possible carriers of the Dutch elm disease has been discussed by
Jacot (15, 16). In some galleries nearly every beetle had one or more
young specimens of Uropoda sp. attached to it. These are probably not
parasitic, but are merely being transported about. They may be attached
to the abdominal sternites or the elytra.

Attached to the intersegmental membrane behind the prosternum are
often found several specimens of Pediculoides dryas Yitzthum. These are
apparently actual parasites but do not seem to be as injurious to their
host as the congener P. venlricosus is to its hosts. Sometimes the mites
are seen eating the eggs, and occasionally the larvae of the beetle. In the
case of the latter, since mites have been found only on dead larvae, it is
not clear whether they actually killed the larvae or are merely saprozoic.

ASSOCIATED FAUNA

Besides those already referred to as predators or parasites of the native
elm bark beetle, there are numerous other insects associated with it under
the bark, and a list has been published (Kaston, 19).

Perhaps the most important associate is the smaller European elm
bark beetle, Scolytus multistriatus Marsham. In this species the breeding
habits are quite similar to those of H. rufipes. They differ in that the egg
galleries of the former are parallel to the grain of the wood, the larval
galleries in general lie across the grain, and the galleries score the wood
surface much more than do those of the native beetle. The European
species tolerates a greater degree of dryness and heat, as evidenced by its
being found more commonly than the latter on the upper sides of branches
in which the native beetle occurs, as well as in branches of smaller diameter.

The elm snout beetles, Magdalis barbita Say and M. armicollis Say, are
fairly common, especially in the smaller, drier limbs.

Probably the most common and abundant insect found in elm bark is
the cerambycid, Saperda tridentata. The larvae may be found in the very
moist bark of a limb or tree which has just recently died, as well as at the
other extreme where the bark is too dry for any scolytids. It competes
successfully with H. rufipes in consuming bark for food and is thus a factor
in keeping down the numbers of the latter species.

In old bark from which most of the H. rufipes have emerged, the larva
of the melandryid, Synchroa punctata Newman, is quite common. It
presumably feeds on the decaying bark.

SUMMARY

The native elm bark beetle, Hylurgopinus rufipes (Eichhoff), is known
to occur over most of the eastern and middle western United States. In
Connecticut there are most commonly one and a half generations per year.

Summary 37

Overwintering larvae give rise to beetles whose progeny hibernate during
the following winter as adults. Likewise, overwintering adults give rise to
beetles whose progeny hibernate the next winter as larvae. There may
also be two generations per year. In a few cases only one generation occurs,
especially when larvae in crowded bark are unduly retarded in their
development. Eggs do not survive the winter, and only a very few indi-
viduals hibernate in the pupal stage.

For breeding purposes the beetles prefer shaded limbs over two inches
in diameter which have not lost more than half their original water content.
The majority of the egg galleries are "V" shaped, biramous, and tend to
run across the grain. The average length appears to be 30 millimeters, and
the average number of eggs per female about 60. Mortality from various
causes reduces the brood so that only about 15 young beetles emerge per
family. Parent beetles raise only a single brood and die in the egg gallery.

Ordinarily, eggs hatch in from 6 to 12 days. The larval stage varies in
length from a minimum of 29 days, at 24.5° C, to about 40 or 50 days,
in the field during the summer months. It has been shown that the
number of instars may vary but it is believed that the usual number is
five or, less commonly, six. It is not possible to determine the number
by the application of Dyar's Law to mixed lots of larvae taken at random
from various localities in the field. The larval galleries tend to run with
the grain and the rate of tunneling was found to be quite variable with
different larvae.

The pupal period of 8 to 12 days follows a quiescent prepupal period of
two or three days. Emerging young beetles fly to healthy elms to feed in
bark tunnels before seeking material in which to breed. There is evidence
to indicate a flight range of at least a mile, though presumably the beetles
will attack the nearest available material. The bark tunnels are here con-
sidered analogous to the well-known crotch and twig feeding injuries of
Scolytus multistriatus Marsham, and hence important in relation to the
spread of the Dutch elm disease. They are in the bark alone, and do not
touch the surface of the wood. Those made for hibernation tend to be
longer and are more often in thicker bark than the otherwise similar
feeding tunnels.

Low temperatures in winter have little effect on the hibernating larvae,
but only about half of the hibernating adults survive. Dryness and com-
petition, both with other H. rufipes larvae and the larvae of Saperda tri-
dentata, probably account for the high mortality during the growing season.
Predators and parasites appear to be of little significance.

38 Connecticut Experiment Station Bulletin 420

BIBLIOGRAPHY

1 . Becker, W. B. Some observations on the overwintering habits of the American elm

bark beetle, Hylurgopinus rufipes Eichh. Jour. Econ. Ent., 28: 1061-1065. 1935.

2. Some observations on the larval instars of Hylurgopinus rufipes in

Massachusetts. (Unpublished) Fourteenth Conf. of Connecticut Entomologists
(New Haven). 1937.

3. Bedard, W. D. Number of larval instars in Denrl melon us. Jour. Econ. Ent., 26:

1128-1134. 1933.

1. Blackman, M. W. Observations on the life historv and habits of Pitvogenes hop-
kinsi Swaine. N. Y. State Coll. For. Tech. Bui. No. 2, Vol. 16: 11-66. 1915.

5 Chapman, J W. The introduction of a European Scolytid (the smaller elm bark-
beetle, Scolylus mullistriatus Marsh.) into Massachusetts. Psyche, 17: 65-67.
1910.

6. Coliins, C. W. Insect vectors of the Dutch elm disease. Proc. Eleventh Nat. Shade
Tree Conf. Philadelphia, 127-132. 1935.

Two elm Scolytids in relation to areas infected with the Dutch elm

disease fungus. Jour. Econ. Ent., 31: 192-195. 1938.

8. Collins, C. W., W. D. Buchanan, B. B. Whitten, and C. H. Hoffmann. Bark

beetles and other possible insect vectors of the Dutch elm disease Ceratoslomella
ulmi (Schwarz) Buisman. Jour. Econ. Ent., 29: 169-176. 1936.

9. Felt, E. P. Balloon drift and insect drift. Ent. News, 48: 17. 1937.

10. Balloons as indicators of insect drift and of Dutch elm disease

spread. Bartlett Tree Bes. Lab., Bui. No. 2: 3-10. 1937.

11. — — Dissemination of insects by air currents. Jour. Econ. Ent., 30:

458-461. 1937.

12. Felt, E. P., and Bromley, S. W. Shade tree insects and spravs, 1937. Jour. Econ.

Ent., 31: 174. 1938.

13. Gaines, J. C, and Campbell, F. L. Dyar's Bule as related to the number of instars

of the Corn Ear Worm, Heliolhis obsoleta (Fab.), collected in the field. Ann.
Ent. Soc. America, 28: 445-461. 1935.

14. Hopkins, A. D. Catalog of W. Va. Scolytidae and their enemies. W. Va. Agr.

Expt. Sta. Bui. 31, Vol. 3: 121-168. 1893.

15. Jacot, A. P. Acarina as possible vectors of the Dutch elm disease. Jour. Econ. Ent.,

27: 858-859. 1934.

16. Three possible mite vectors of the Dutch elm disease. Ann. Ent.

Soc. America, 29: 627-635. 1936.

17. Kaston, B. J. The morphology of the elm bark beetle, Hylurgopinus rufipes

(Eich.). Conn. Agr. Expt. Sta., Bui. 387: 613-650. 1936."

18. — — ■ — Notes on Hvmenopterous parasites of elm insects in Connecticut.

Conn. Agr. Expt. Sta., Bui. 396: 351-361. 1937.

19. — ■ Check list of elm insects. Conn. Agr. Expt. Sta., Bui. 408: 235-

242. 1938.

20. Kaston, B. J., and Becker, W. B. Spathius canadensis Ashm., a parasite of Hylur-

gopinus rufipes (Eich.). Jour. Econ. Ent., 29: 807. 1936.

21. Kaston, B. J., and Biggs, D. S. Studies on the larvae of the native elm bark beetle.

Jour. Econ. Ent., 30: 98-108. 1937.

22. On certain habits of elm bark beetles. Jour. Econ. Ent., 31: 467-

469. 1938.

Bibliography 39

23. Keen, F. P., and Furniss, R. L. Effects of subzero temperatures on populations of

western pine beetle. Jour. Econ. Ent., 30: 482-504. 1937.

24. Martin, C. H. Preliminary report of trap-log studies on elm bark beetles. Jour.

Econ. Ent., 29: 297-306. 1936.

Field notes on the life history of Hylurgopinus rufipes (Eich.).

Jour. Econ. Ent., 31: 470-177. 1938.

26. Metcalfe, M. E. On a suggested method for determining the number of larva! in-

stars in Sitodrepa panicea L. Ann. Appl. Biol., 19: 413-419. 1932.

27. Oldham, J. N. Nematode parasite of Scolytus scolylus, and Scolylus multislriatus,

Parasitylenchus scolyti n. sp. Jour. Helminth., 8: 239-248. 1930.

28. Prebble, M. L. The larval development of three bark beetles. Can. Ent., 65: 145-

150. 1933.

29. Robinson, W. The thermocouple method of determining temperature. Ann. Ent.

Soc. America, 20: 513-521. 1927.

30. Determination of the natural undercooling and freezing points in

insects. Jour. Agr. Res., 37: 749-755. 1928.

31 . Steiner, G. Some nenhc parasites and associates of the mountain pine beetle

(Dendroctonus monticolae). Jour. Agr. Res., 45: 437-444. 1932.

32. Struble, G. R. The biology of certain Coleoptera associated with bark beetles in

western yellow pine. Univ. California Publ. in Ent., 5: 105-134. 1930.

33. Tragardh, I Studies on the galleries of the bark beetles. Bui. Ent Res., 21: 469-

480. 1930.

The following papers, having to do in whole or in part with //. rufipes, are
included here though not referred to either in the body of this bulletin or
in the previous one on morphology.

Becker, W B. Some notes on the tunneling habits of Hvlurgopinus rufipes Eich.
Jour. Econ. Ent., 30: 375. L937.

Becker, W. B., and Tomlinson, W. E. Distribution of elm bark beetles in Massa-
chusetts. Jour. Econ. Ent., 31: 323. 1938.

Britton, W. E. Another probable carrier of the Dutch elm disease. Conn. Agr.
Expt. Sta., Bui. 368: 256-257. 1935.

Britton, W. E., and Friend, R. B. Insect pests of elms in Connecticut. Conn. Agr.
Expt. Sta., Bui. 369: 298. 1935.

Chapman, J. W. The leopard moth and other insects injurious to shade trees in
the vicinitv of Boston. Bussey Inst. Publ. Contr. No. 48 from Ent. Lab., 3-51.
1911.

Dietrich, H. Elm bark beetles in N. Y. State. Jour. Econ. Ent., 29: 217. 1936.

Felt, E. P. Bark beetles and the Dutch elm disease. Jour. Econ. Ent., 28: 231-236.
1935.

McKenzie, M. A., and Becker, W. B. The Dutch elm disease. Mass. Agr. Expt.
Sta., Bui. 343: 1-16. 1937.

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Connecticut

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